[SPARK-56505][SQL][TESTS][FOLLOWUP] Make the classic session in SparkSessionBinderBase an overridable seam

[cloud-fan]

What changes were proposed in this pull request?

This is a follow-up to #56190 (which added SessionQueryTest and the SparkSessionBinder /
SparkSessionBinderBase traits).

This PR makes the classic SparkSession used by the test-session lifecycle hooks in
SparkSessionBinderBase an overridable seam, so a suite can bind its session per test instead of
only through the single shared beforeAll session.

SparkSessionBinderBase exposes the bound session through the abstract spark (typed as the
session-agnostic SparkSession, so the connect binder can narrow it to a connect session). Its
per-test afterEach hook, however, needs the classic session (sharedState.cacheManager), and
today reads the trait's private var _spark directly. That is fine for the common case where the
session is created in beforeAll, but it hard-codes the assumption that the shared field is the
session the hooks should act on.

Changes:

• Add protected def classicSpark: classic.SparkSession = _spark to SparkSessionBinderBase and
  route the per-test hook through it (afterEach's cacheManager.clearCache()).
• connect.SparkSessionBinder already computed the same value as a private def classicSpark;
  change it to override protected def classicSpark of the promoted accessor (no behavior change).
• Add PerTestSessionBinderSuite, a small example suite that runs each test in a fresh instance
  (OneInstancePerTest) bound to a per-test cloned session, by overriding classicSpark. This both
  demonstrates the new flexibility and guards the seam.

Why are the changes needed?

The current binder can only bind the session created in beforeAll. A suite that wants an isolated
session per test -- e.g. one mixing in OneInstancePerTest, where ScalaTest runs each test in a
fresh suite instance that executes only beforeEach/afterEach and never beforeAll -- has no way
to tell the inherited hooks which session to operate on; those hooks read the private shared field,
which is null in the per-test instance. Making classicSpark overridable removes that limitation
while leaving the default (shared-session) behavior unchanged.

Does this PR introduce any user-facing change?

No. This is test-only; the default behavior of SparkSessionBinderBase and the connect binder is
unchanged.

How was this patch tested?

New PerTestSessionBinderSuite exercises the per-test binding path and passes. Existing binder-based
suites compile and run unchanged (the connect binder change is a private def -> override protected def with the same body).

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.8)

[cloud-fan]

cc @fwc @MaxGekk @zhengruifeng